A variety of photographic materials are known that can be processed by immersion in various processing solutions or baths. Such materials contain various layers and components, including antihalation or filter layers, overcoats and radiation sensitive layers.
The antihalation layer of an imaging element helps to prevent light that has passed through the radiation sensitive layer(s) from reflecting back into those layers. If reflection is not prevented, the resulting image is less sharp. In wet processes, the antihalation layer is generally removed or rendered colorless during processing.
Imaging elements are also known that can be processed, after imagewise exposure, simply by heating the element. These elements are often known as thermographic or photothermographic elements. It is generally desired that such elements include an antihalation or filter layer, especially to provide improved microimaging capability. In most cases, the antihalation layer must be rendered substantially transparent upon heat processing in order to avoid use of processing solutions.
A variety of antihalation compositions have been reported in the literature whereby the use of processing solutions is avoided. Such compositions generally include particular heat bleachable antihalation dyes, or incorporated addenda that act as bleaching agents.
For example, the use of radicals from biimidazoles in antihalation compositions is known, being described for example in U.S. Pat. No. 4,196,002 (Levinson et al) and U.S. Pat. No. 4,201,590 (Levinson et al). The heat developable compositions described therein become colorless upon exposure to heat for a given time.. The compositions contain formazan dyes used in reactive association with certain hexaarylbiimidazole compounds, which compounds are oxidative dimers of alkyl-substituted-arylimidazoles.
All of the known antihalation compositions suffer from one or more problems, including heat instability and requiring high heat processing or exhibiting only temporary bleaching. There is a continuing need for antihalation compositions that can be permanently and quickly bleached at lower temperatures.
Such compositions are described and claimed in copending and commonly assigned U.S. Ser. No. 08/583,395, having a filing date of Jan. 5, 1996, based on Provisional Application 60/008,156 filed on even date herewith by Perry, Goswami and Zielinski, and entitled HEAT BLEACHABLE ANTIHALATION COMPOSITION, ELEMENTS CONTAINING SAME AND METHOD OF USE. Such compositions include an acidic component in the antihalation composition.
While the invention described in the noted application has provided a significant advance in the art, there are instances when acids cannot be readily incorporated into the antihalation composition. Moreover, it would be desirable to have the flexibility to use acids outside of the antihalation composition that are prone to premature bleaching. Such acids must still be available to participate in the reactions that occur in the antihalation composition during heat processing.